In the case of bulk-material silos with base ventilation, the loosening air or a considerable part thereof has to escape through the silo draw-off opening together with the material. This results in a considerable pressure difference and high speeds prevailing at the draw-off opening. Non-uniform movement of the material within the silo may also result in a non-uniform draw-off. The resulting difficulties are controlled by using a so-called expansion chamber, this being a chamber which is of a considerable volume and considerable height and is provided directly downstream of the silo outflow opening. In this chamber, the material discharged out of the silo is relieved from the silo pressure and the excess air is led away before the material passes the draw-off members. The precondition for the expansion of, and air extraction from, the material is an expansion-chamber height which is greater than the height of expansion rise of the material which passes in the fluidized state out of the silo into the expansion chamber. The height of expansion rise is determined, in accordance with the principle of communicating pipes, by the pressure by which the material passes out of the silo into the expansion chamber. This height is usually in the region of from 4 m to 10 m. The column of material in the expansion chamber gives the excess air opportunity to escape, with the result that a level of material above which the excess air can be drawn off forms in the upper space of the expansion chamber. An important factor for the expansion action is that the expansion chamber follows on directly from the silo outflow opening, in order that the expansion takes effect directly at the silo outflow opening and that there is no counter-pressure, obstructing the outflow, as a result of high flow resistances. In the case of known configurations, this has resulted in the outflow chamber being arranged within the silo region in the main silo space (FR-A-2 350 288, GB-A-20 74 549) or beneath the silo base (EP-A-400 331). This requires a high degree of structural outlay.